This invention relates to apparatus for measuring the actual mechanical power being produced by an electric motor and measuring and monitoring the amount of reactive torque acting on the drive shaft of the electric motor and the rotational speed of the drive shaft. It relates more particularly to a motor power measuring cell useful in measuring and monitoring the actual mechanical power produced by an electric motor and the amount of reactive torque acting upon the drive shaft and the rotational speed of the drive shaft of the electric motor used to actuate a motor operated valve. This invention is especially useful in the field of valve diagnostics.
In the nuclear power industry, valves are actuated remotely by electric motors to open, closed or intermediate positions to improve or maintain power plant output and to maintain proper cooling of the nuclear reactor. Continuous and proper operation of the valves is essential for reliable operation of the power plant and the safety of the general public. Both the power industry and the valve industry have had a need to develop apparatus and systems to continuously measure, test and monitor various operating parameters associated with valves, especially remote motor operated valves, and to alert the power plant operator of any significant changes in any of the operating parameters being measured, tested and monitored.
U.S. Pat. No. 4,542,649 to Charbonneau et al. discloses a recently developed motor operated valve monitoring system that measures, records and correlates a number of valve operating parameters, such as valve stem load, valve stem movements, valve stem torque, spring pack movement and motor current. The information provided by the system described in the Charbonneau et al. patent provides a direct indication to the operator of any valve problems, such as excessive or inadequate packing loads, excessive inertia, incorrectly set limit and torque switches, malfunctioning thermal overload devices, excessive stem thrust loads, gear train wear and damage to the valve stem.
As a result of the teachings of the Charbonneau et al. patent, efforts have been made by ITI Movats, Inc., the assignee of the Charbonneau et al. patent, as well as the assignee of the present patent application, to develop additional apparatus and systems by which the operation and performance of motor operated valves could be easily measured, tested and monitored by an operator at a location remote from the valve. Several such systems have been developed for monitoring AC motor operated valves by measuring and monitoring electrical power parameters of the AC motor during its operation and are described in U.S. Pat. Nos. 4,787,245; 4,831,873; 4,869,102; 4,891,975 and 5,000,040 assigned to Movats, Inc. Another system has been developed for monitoring DC motor electrical power parameters for DC motor operated valves and is described in U.S. Pat. No. 4,888,996 assigned to ITI Movats Inc. This system measures and monitors motor armature current, voltage drop across the armature and the electrical resistance of the armature and uses the measurements to calculate output power of the motor, armature speed and a motor torque factor representative of the motor output torque multiplied by a constant. There has been a need to develop apparatus and systems to continuously and directly measure and monitor the actual mechanical power being produced by an electric motor by continuously and directly measuring both the rotational speed and the total reactive torque acting on the drive shaft of the electric motor used to operate a motor operated valve (MOV), while the valve is in service. Prior to this invention, such continuous and direct mechanical drive shaft rotational speed and total reactive torque measurements have not been possible with commercial MOVs since the electric motor is sealed in a housing that is fastened directly to the sealed housing of the valve actuator. The commercial MOVs do not normally have an exposed drive shaft or other means to attach the sensors needed to mechanically measure the rotational speed or total reactive torque acting on the drive shaft.
The rotational speed and torque supplied by the drive shaft of a motor could be measured by removing the motor from the MOV assembly and testing the motor in a test stand but this type of bench test does not provide measurements or information concerning the actual mechanical power of the motor or the actual rotational speed or reactive torque acting on the motor drive shaft at all times while the motor operated valve is actually in service.